The natural crystalline lens of the eye plays a primary role in focusing light onto the retina for proper vision. However, vision through the natural lens may become impaired because of injury, or due to the formation of a cataract caused by aging or disease. To restore vision, the natural lens is typically replaced with an artificial lens. An artificial lens may also be implanted as a replacement or a supplement to the natural lens in order to make a refractive or other vision correction.
A natural lens is generally removed through the use of a slender implement which is inserted through a small incision in the eye. The implement includes a cutting tool that is ultrasonically vibrated to emulsify the lens. The emulsified fragments of the lens are aspirated out of the eye through a passage provided in the cutting tool. The slender nature of the implement enables extraction of the lens through a small incision in the eye. The use of a small incision over other procedures requiring a large incision can lessen the trauma and complications experienced during surgery and postoperatively.
The artificial lens is composed of a flexible material so that the lens can be folded and/or compressed to a smaller cross-sectional size, and thus avoid enlargement of the incision during implantation of the lens. To this end, inserters ordinarily include a lens reducing structure which functions to reduce the cross-sectional size of the lens, and a cannula with a lumen to direct the lens into the eye. The lens reducing structure has taken many different forms including, for example, hinged sections which close about a lens and tapering lumens which compress the lens as it is advanced toward the eye. The cannula is a slender, thin-walled tube at its distal end that guides the lens through the incision and into the eye. The lumen along the distal portion of the cannula generally has a substantially uniform configuration and size (i.e., with only a slight taper for molding purposes) to avoid additional high forces needed to further compress the lens. By maintaining a substantially uniform lumen, the risk of rupturing the thin walls is alleviated.
While there is great interest in making the distal end of the inserter as narrow as possible, there are practical considerations which have limited the extent to which the size of the cannula can be reduced. For instance, as mentioned above, large inwardly directed forces are needed to further reduce a lens which is already tightly compressed. As a result, merely reducing the diameter of the lumen at its distal end to achieve a smaller cannula will at some point increase the inwardly directed forces so as to impede the advance of the lens or cause rupture of the walls. Also, further thinning of the walls to reduce the cannula without narrowing the lumen will also at some point lead to rupture of the cannula walls during use.